Fixes #23733.

What & Why

_build_api_kwargs (run_agent.py) had two parallel return paths for chat_completions:

• Legacy branch (get_provider_profile() → None, unknown provider): called _prepare_messages_for_non_vision_model to strip image_url parts before they reached non-vision models.
• Profile branch (registered providers — opencode-go, deepseek, kimi, openrouter, gemini, etc.): did not call it, so image_url parts passed through to the upstream provider.

Result: text-only models on any registered provider 400'd with unknown variant 'image_url' whenever a multimodal request reached /v1/chat/completions (and the same path from any caller that lets the agent build messages from images). The gateway-adapter path (gateway/run.py → _decide_image_input_mode → _enrich_message_with_vision) and the codex_responses profile path both already handle this; the chat_completions profile branch was the lone gap.

The fix mirrors the legacy branch: one call to _prepare_messages_for_non_vision_model(api_messages) immediately before _ct.build_kwargs(...) in the profile branch. The function early-returns when no message contains image parts and again when _model_supports_vision() is true, so the added call is free on the common paths.

How to test

# The targeted regression tests (new + existing):
uv run python -m pytest tests/run_agent/test_vision_aware_preprocessing.py -v
# 13 passed (11 pre-existing + 2 new in TestBuildApiKwargsProfileBranch)

# Adjacent test directories — full pass, no regressions from this change:
uv run python -m pytest tests/run_agent/ tests/agent/test_image_routing.py \
    tests/agent/transports/test_chat_completions.py tests/providers/ -q

The added TestBuildApiKwargsProfileBranch covers both directions on the profile path:

• test_profile_branch_strips_images_for_non_vision_models — fails on main, passes with the fix. Sets provider=opencode-go + model=deepseek-v4-pro, calls _build_api_kwargs with an image_url user message, and asserts the messages handed to the transport contain no image_url parts.
• test_profile_branch_passes_through_when_model_supports_vision — already-passes guard so vision-capable models don't regress to text-only.

Platforms

Tested on Linux (Debian trixie). Change is pure Python message-shape manipulation; no file I/O, process management, or terminal handling.

Notes

The full root-cause analysis (including verification on origin/main 64145a199) is in the linked issue.